Abstract:
Maghemitization of magnetite is an important topic in rock magnetism. It happens as low-temperature oxidation (LTO) under normal atmospheric conditions, causing gradual transformation of magnetite to maghemite, and potentially culminating in transformation to hematite. Better understanding of how humidity and temperature affect LTO is of great interest for paleoclimate studies, which use magnetic properties as proxies.
A laboratory experiment was conducted under controlled humidity and temperature conditions. Natural magnetite powder samples were exposed to different relative humidity (rH) at room temperature (room humidity and rH >90 %) and at 70 °C (rh of 5 %, 13 %, >90 %). After one year, the magnetite sample that was exposed to highest relative humidity (>90 %) at 70 °C showed the strongest increase in the oxidation degree by ~3 % according to Verwey transition temperature (Tv), and it was the only setup in which hematite was detected by Mössbauer analysis. Other setups at 70 °C also showed a measurable change in the degree of LTO, but the dependence on humidity was found to be non-linear. For better understanding the influence of humidity and temperature on magnetite oxidation in natural environments, magnetic properties of basalts and weathered pebble samples directly above the fresh rock were studied. Basalts and pebble samples were collected from the Deccan traps in India along a gradient of mean annual precipitation (MAP) between ~500 and ~4000 mm, and from the Emeishan traps in SW-China allowing a comparison of alteration at approximately same MAP (~1000 mm) but ~15 °C different mean annual temperatures. The selected basalts contain magnetite that probably derived from exsolution during cooling. The weathered pebbles were divided into five groups by their grain size, and an increasing tendency of alteration is expected with grain size fining. The results suggest no systematic humidity-related changes of magnetic parameters between basalts and their weathered pebbles. Combining the results of both the laboratory experiment and the study of natural basalts, a humidity and temperature influence on the degree of oxidation is evidenced, however, it does not occur in a linear way, and the initial rock composition seems to mask the climate-related alteration effects in nature. Further studies on natural rock settings with other rock types and laboratory experiments with more variable humidity and temperature conditions are needed.
LTO could also play a role when material is translocated from the source to a sink. Soil and lake sediments archives often preserve important paleoclimate signals. In a source-sink setting, in which soil contributes as the major source for lake sediments or basin deposits, the possible changes of the properties of magnetic minerals during transport are important factors for interpreting magnetic properties in the sink material as climate proxies. A study was conducted, in which the only important magnetic source material delivered to a small lake (Caohai Lake) is highly magnetic red soil with susceptibilities (χ) of ~10-5 m3/kg, dominated by pedogenic highly maghemitized nano-magnetite (~10-15 nm) arranged in aggregates of ~100 nm, causing superparamagnetic (SP) behavior that tails into the stable single-domain (SSD) range. Partial disintegration of the aggregates and increasing alteration of the magnetite nanoparticles to hematite during transfer of the source material to Caohai Lake was interpreted from transmission electron microscopy results and frequency dependence of χ. The results indicate that the contribution of magnetite nano-particles to the magnetic properties of lake sediments diminishes by disintegration of aggregates and increased magnetite-to-hematite transformation and may even become neglectable compared to the larger sized detrital magnetic fraction. The ratio of saturation magnetization and χ is suggested as a useful proxy in red soil, caused by climate-related initial LTO degree in the nano-particles of the aggregates.
